Can't speak for OP, but maybe they mean things like Intel Management Engine, or cryptographicly locked bootloaders (not a backdoor, obviously, but a nuisance) with keys we can't control. So the backdoor is "feature", not a conspiracy.
Of course, you could avoid manufacturers that put this type of security crap in their chips. But with that logic you can already avoid Intel ME by using CPUs from Via...
(The point I'm trying to make is that an open ISA doesn't really solve all our problems. I feel Berkeley folks are overpromising here)
When we talk about security on Intel CPU, it's tragic. When we talk about security about RISC-V, it's a complex problem. That's a massive improvement. But yes, the work is far from over.
I'm not a fan at all of cryptocurrencies, but there are some interesting cryptographic ideas behind Polkadot (with Jeff Burdges) and Golem Project (with Joanna Rutkowska) to use those networks security to bring trust back to the laptop/phone level.